Raman spectroscopy is gaining widespread acceptance as a chemically specific identification tool, as manifested by the rapid growth of handheld analyzers in the past decade. A commonly used method to identify unknown material is searching through spectral libraries by means of hit quality index (HQI). Frequently the test spectra and the library spectra are acquired on different instruments. For meaningful comparison, such spectra must be intensity corrected to calibrate out the unit to unit variations in spectral response. Earlier methods developed to standardize relative Raman spectral intensities used white light irradiance sources. McCreery's group pioneered the work of using luminescent glass as a convenient source that can easily reproduce the sampling condition. This methodology was later adopted by the National Institute of Standards and Technology, which now produces certified luminescent glass relative intensity standards for various excitation wavelengths. Both white light and luminescence standards are used presently in research and commercial instrumentation, and have facilitated meaningful comparison of Raman spectra acquired on different instruments, even of different excitation wavelengths. Consequently, commercial Raman spectral libraries are all standardized with relative intensity correction.
To match with the standardized library spectra, the sample spectrum acquired on a test instrument must also be corrected for the instrument response. This is a widely accepted practice, and is incorporated in most commercial Raman library search algorithms. However, there is a side effect with this approach, that is, the intensity correction magnifies the contribution to the HQI from spectral regions of lower responsivity, where the signal to noise ratio (SNR) is often poor. As will be demonstrated in this manuscript, this results in reduced specificity. This effect is particularly prominent in handheld analyzers using NIR excitations and CCD detectors, where the instrument response often varies greatly across the spectral range, and SNRs in the low responsivity regions are typically poor.